The invention relates to a method for the speed control of a variable-speed wind turbine generator system by varying the angle of blade attack and torque for the process of switching it into the grid.
Variable-speed wind turbine generator systems including torque governors and governors of the angle of blade attack are known. For a control of the angle of blade attack, each rotor blade is adjusted in its angle of blade attack about its longitudinal axis. If the angles of blade attack vary the rotor blade will take a different torque each from the wind.
For a control/regulation of wind turbine generator systems, it is known to distinguish between two modes of operation. The first mode of operation is referred to as partial-load operation in which a speed control is performed via a predetermined torque. The second mode of operation is full-load operation in which a speed control is performed via an adjustment of the angle of attack.
The speed of the system during a partial-load operation is set to the optimum relationship between the rotor circumferential speed and wind speed (λopt) to achieve an optimum power output by the wind turbine generator system. At this time, the rotor blades are set to a blade angle which generates the highest drive torque for the rotor shaft. In the partial-load range, the speed of the rotor is adjusted via the reaction torque as generated on the generator and/or converter.
If the maximum reaction torque is reached on the generator at a rated speed the speed can no longer be maintained at the working point by a further increase to the generator torque. Then, an overload is avoided by deteriorating the aerodynamic efficiency of the blades and taking them out of the optimum angle of attack. Hence, once the maximum generator torque is attained the speed of the rotor is acted on via the angle of blade attack.
To enable an operation of a wind turbine generator system in the partial-load or full-load modes, it is necessary first to switch the system into the grid. A reaction torque cannot be generated on the generator before because currents cannot be impressed onto the generator until there is a connection to the grid. Therefore, as a rule, the rotor of the wind turbine generator system will rotate freely prior to switching it into the grid and an electric connection is established by switching it into the grid and the wind turbine generator system starts getting controlled for a feed of electric energy to the grid.
The switch-in procedure necessitates a certain minimum speed of the generator coupled to the rotor; the so-called synchronization speed nsync.
From EP 1 007 844 B1, the entire contents of which is incorporated herein by reference, a variable-speed wind turbine generator system is known which has an induction generator with a wound-coil rotor. For operation management, the known wind turbine generator system is known to have a torque control and a control of the angle of blade attack which works independently of the torque control.
In the known wind turbine generator system, the control of the angle of blade attack adjusts a very large angle of blade attack of 25°, for example, below a certain speed of the generator that is very low and is far below the synchronization speed. This helps ensure that even if the rotor speeds are very low a sufficient torque is taken from the wind in order to set the rotor into motion. Once the predetermined speed, which is very low, is achieved for the generator an angle of blade attack is set which matches with the optimum angle of blade attack for operation in the partial-load range.
Accordingly, in the known wind turbine generator system, a control of the speed via the angle of blade attack is not provided in that speed range in which the switch-in procedure takes place because the angle of blade attack has assumed a fixed value already which was optimized for the partial-load range. Since it is impossible either to influence the speed by predetermining a torque on the generator prior to the switch-in procedure the speed will not be regulated at all prior to the switch-in procedure.
Therefore, it is the object of the invention to provide a method for the operation of a variable-speed wind turbine generator system that improves the switch-in procedure.